The blazar TXS 0506+056 associated with a high-energy neutrino: insights into extragalactic jets and cosmic ray acceleration

TL;DR

This study links a high-energy neutrino detection to the blazar TXS 0506+056, using multiwavelength observations and a novel lepto-hadronic model to explore jet physics and cosmic ray acceleration.

Contribution

It introduces a new one-zone lepto-hadronic model that explains multi-messenger signals from TXS 0506+056, connecting neutrino and electromagnetic observations.

Findings

Reproduces multiwavelength spectra with neutrino compatibility.

Supports a jet power of approximately 10^{45} to 4×10^{46} erg/s.

Suggests protons can reach energies up to 10^{18} eV in the jet.

Abstract

A neutrino with energy of $\sim$290 TeV, IceCube-170922A, was detected in coincidence with the BL Lac object TXS~0506+056 during enhanced gamma-ray activity, with chance coincidence being rejected at $\sim 3\sigma$ level. We monitored the object in the very-high-energy (VHE) band with the MAGIC telescopes for $\sim$41 hours from 1.3 to 40.4 days after the neutrino detection. Day-timescale variability is clearly resolved. We interpret the quasi-simultaneous neutrino and broadband electromagnetic observations with a novel one-zone lepto-hadronic model, based on interactions of electrons and protons co-accelerated in the jet with external photons originating from a slow-moving plasma sheath surrounding the faster jet spine. We can reproduce the multiwavelength spectra of TXS 0506+056 with neutrino rate and energy compatible with IceCube-170922A, and with plausible values for the jet power of $\sim 10^{45} - 4 \times 10^{46} {\rm erg \ s^{-1}}$. The steep spectrum observed by MAGIC is concordant with internal $\gamma\gamma$ absorption above a few tens of GeV entailed by photohadronic production of a $\sim$290 TeV neutrino, corroborating a genuine connection between the multi-messenger signals. In contrast to previous predictions of predominantly hadronic emission from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton up-scattering of external photons by accelerated electrons. The X-ray and VHE bands provide crucial constraints on the emission from both accelerated electrons and protons. We infer that the maximum energy of protons in the jet co-moving frame can be in the range $\sim 10^{14}$ to $10^{18}$ eV.